Cavity type particle stream accelerator having a non-isothermal chamber



H. HUBER. ETAL 3 346 750 1967 CAVITY TYPE PARTICLE STREAM ACCELERATOR HAVING A NON-ISOTHERMAL CHAMBER Filed March 1, 1966 I I .Hi wfi "WI 7 i Q 714 l 6 g G 3 12 13 H- HUBER e/ .D-MAUGIS By Qfl 0 52;.

ATTORNEY United States Patent M CAVITY TYPE PARTICLE STREAM ACCEL- ERATOR HAVING A NON-ISOTHERMAL CHAMBER Harry Huber and Daniel Maugis, Paris, France, assignors to CSF-Compagne Generale de Telegraphic Sans Fil, a corporation of France Filed Mar. 1, 1966, Ser. No. 530,899 Claims priority, application France, Mar. 1, 1965,

Claims. to]. 313-231 ABSTRACT OF THE DISCLOSURE A source of electrons or ions of the cavity type filled with a plasma of ionizable substance such as cesium vapor wherein volume ionization of the vapor is enhanced by creating conditions for a thermal arc discharge within the cavity. Such conditions are created by making the walls of the cavity non-isothermal.

The present invention relates to sources of charged particles which may be either ions or electrons emerging from an orifice in a wall of a cavity formed by a hollow body. More particularly the instant invention relates to those sources in which the cavity is filled with a plasma of an ionizable substance, such as cesium vapor, while the electronic space charge is substantially neutralized in the interior of the cavity. Such sources, referred to as hollow cathodes when operating as emitters of electrons, have been described in the U.S.A. patent application Ser. No. 216,141, filed on Aug. 10, 1962, now US. Patent No. 3,253,180, for Hollow Cathodes, by Harry Huber and assigned to the assignee of the present application.

In this prior application the cavity was isothermal and all the walls thereof were made of a common material. The electrons emitted by the internal walls of the cavity mixed with cesium ions supplied through a tubulure connecting the cavity to a reservoir of liquid cesium. These ions were formed by the surface ionization phenomenon. By disposing a positive electrode in front of.the outlet orifice of the cavity, a stream of electrons was extracted from the plasma. Likewise it was possible to extract from the source a stream of ions by disposing in front of the outlet orifice a negative electrode. In either case a stream of particles of a relatively high density was obtained.

However, practice has shown that this prior art source presents various inconveniences, due mainly to the incomplete ionization of the cesium in the cavity, that is, due to the presence in the plasma of a relatively important amount of neutral atoms.

When electrons are being extracted, these neutral atoms eifuse through the outlet orifice and are ionized outside of the cavity by the electrons which emerge from the orifice at the same time as the neutrons. An ion sheath is thus formed in the space between the external walls of the cavity and the electrode which accelerates the electrons. The ion sheath renders this space conductive and brings about an arc discharge between the cavity and the positive electrode, resulting in a short-circuit of this electrode and rendering the device inoperative.

3,346,750 Patented Oct. 10, 1967 When ions are being extracted, the neutral atoms, eiiusing like in the previous case through the outlet orifice, mix with the ion stream and become a nuisance when the source is to be utilized, for example, for ionic propulsion, because of the known effect of erosion on the electrodes due to the impact of these neutral particles which escape any focusing action. In the US. patent application Ser. No. 447,792, filed on Apr. 13, 1965, for Ion Source, by R. Le Bihan and D. Maugis and assigned to the Assignee of the present application, it has already been proposed to reduce the number of neutral atoms in the extracted ion stream by reducing the dimension of the orifice to a few tens of microns (i.e. to a diameter less than the Debye thickness), but this solution presents the drawback of reducing the intensity of the ion stream or current.

The present invention has for its object a cavity type generator for producing a stream of charged particles in which the aforementioned inconveniences are avoided, that is, preventing the escape of neutral atoms without reducing the dimension of the outlet orifice, thereby fully conserving the advantage of a high particle density usually yielded by plasma filled sources.

In accordance with the present invention, the ionization degree of the ionizable substance in the interior of the cavity is improved, and the neutral atoms are reduced in number or even entirely suppressed, by creating in the cavity conditions of a thermal arc discharge which favors volume ionization.

The means for producing these conditions of an internal arc discharge in accordance with the present invention comprise a non-isothermal cavity whose walls are made either of a single, common material or of two different materials having diiferent work functions. In that latter case, the portion or portions of the material having a higher work function are set to a higher tem perature. Moreover, the parameters such as the nature and vapor pressure of the ionizable substance, the nature and temperatures of the materials, the spacings between opposite walls, etc. are chosen in such a way that an internal arc discharge is set up in the cavity.

The present invention will be understood more fully from the following description when taken in connection with the accompanying drawings, in which:

FIGURE 1 is an axial cross-sectional view through a first embodiment of the present invention,

FIGURE 2 is an axial cross-sectional view through a second embodiment thereof,

FIGURE 3 is an axial cross-sectional view through an embodiment provided with a developed profile, and

FIGURE 4 is an axial cross-sectional view through a further embodiment, according to the present invention.

Referring to the drawing, wherein like reference numerals are used throughout the various views to designate like parts, and more particularly to FIGURE 1, there is represented a cavity source of particles shaped as a flat cylinderical box, comprising an upper cover 1 pierced by an outlet orifice 2, a lower cover 3, and a side wall 4. The inner surface of the box may be coated, at least partly, with an electron emissive layer of conventional type. Disposed in front of the outlet orifice 2 is a focusing electrode 5, set at a suitable focusing potential and an accelerating electrode 6, set at a positive or negative potential with respect to the cavity according to whether ions or electrons are to be extracted from the cavity. The Wall 3 is pierced by an orifice 7, through which a tubulure 8 establishes an intercommunication with a reservoir of cesium 9, heated to a suitable vapor temperature by conventional means 11.

In accordance with the present invention, the walls of the cavity are maintained in non-isothermal conditions, for example, by concentrating the conventional heating means 12 only schematically indicated in (FIG. 1) near the lower wall 3. The walls 1 and 3 may be made of different materials; in that case the material of wall 3 is chosen with a higher work function for a given temperature in the presence of cesium vapor at a given temperature. But the Walls 1 and 3 can also be made of one and the same material; in this case the entire box with the side wall 4 is made of a single material.

Let it be assumed, for example, that the cesium vapor is brought to the temperature of 600 K., which corresponds to a pressure in the enclosure of the order of 4 Torr. Several examples can then be indicated for the choice of materials.

First example: wall 3tungsten at 1900 K.; wall 1 tungsten at 1200 K. In these conditions, for the indicated ambiance, there is established an output voltage of 1.2 volt between the two walls.

Second example: wall 3-tantalum at 1900 K., wall 1tungsten at 1200 K. Output voltage-1.5 volt.

Third example: wall 3--rhenium at 2500 K., wall 1molybdenum at 1900 K. Output voltagevolt.

Moreover, the spacing between walls 1 and 3 is chosen in such a manner that in the conditions defined in any of the above examples, a thermal discharge are is set up in the interior of the cavity. From investigations on plasma diode converters it is known that a suitable spacing is comprised between 0.2 and 2 mm.

Operation This device operates as follows:

The electrons emitted from the heated cavity walls behave as preionizing electrons producing a certain degree of volume ionization in the cesium vapor which arrives from reservoir 9 through tubulure 8 and fills the cavity. This excited vapor produces a circulation of free electrons, accelerated by the work function differ ence between the opposite walls 1, 3 of the cavity. The collisions between the electrons and excited atoms bring about the electron avalanche phenomenon resulting in the setting up of a thermal arc discharge, especially in the hatched region 13. The intense ionization produced by this are in turn releases a great number of free electrons Whose collisions with other neutral atoms achieve the ionization of the latter. The interior of the cavity then fills with a plasma having a high degree of ionization.

The accelerating electrode 6, according to its potential, then extracts from the cavity a stream of either ions or electrons, represented schematically at 14, which is substantially pure.

Experiments, carried out by Applicants, have shown that with a device as described and assuming an ionization degree of 10- it is possible to obtain a current of either cesium ions with a density of 0.8 amp/crnF, or electrons with a density of 400 amp/cm FIGURE 2 represents the cavity provided with the cesium reservoir and designated by the same reference numerals as in FIGURE 1, from which it distinguishes only by the fact that the heating means 12 are concentrated around the upper wall 1 which therefore becomes hotter than the lower wall 3. Consequently, the choice of materials indicated in the examples quoted above must be reversed for the walls 1 and 3, that is the work function of wall 1 must be higher than that of wall 3. Operatively, this disposition differs from the previous one only by the direction of circulation of the accelerated electrons which are free in the plasma, the final result remaining unchanged.

In the embodiment of FIGURE 3, wherein again the same reference numerals designate the same elements as in the previous figures, one of the two walls which serve as electrodes, for example, wall 3 is provided around the central orifice 7 with a re-entrant lip 15 whose extremity is spaced apart from wall 1 by a spacing suitable to set up an are within region 13. This disposition presents the advantage of more clearly defining and localizing the region affected by the arc.

The disposition of FIGURE 2 for the heating means and the choice of materials may of course be applied also to FIGURE 3.

In the embodiment of FIGURE 4, the cesium conduit tubulure 8 communicates with the cavity through the side wall 4 instead of through the bottom 3, in which the orifice 7 is consequently suppressed. A lip 15 is provided on the wall 3 like in FIGURE 3. The space available below the bottom 3 is utilized for placing therein a filament 16 which is equivalent to the heating means 12. Otherwise, the choice of parameters and the operation are the same as in the previous figures.

The present invention is not limited to the examples described and illustrated, but admits, on the contrary, of all changes accessible to those skilled in the art without modifying the general characteristic features above-described. Consequently, we do not wish to be limited to the details shown and described herein but intend to cover all such changes as are encompassed by the scope of the appended claims.

We claim:

1. A source of charged particles of the type having a heated cavity in which electrons are emitted from at least a portion of the inner surface of the cavity, while a plasma of an ionizable substance is produced by contact ionization within the cavity, said cavity comprising two mutually facing, opposite walls, spaced apart by an intermediate wall and heated to two different temperatures, the spacing between said opposite walls being sufficiently small to enable an arc discharge at least within a portion of the space comprised between said opposite walls.

2. The source of particles according to claim 1, wherein said opposite walls are substantially parallel.

3. The. source of charged particles as claimed in claim 1, wherein said opposite walls are made of a common material.

4. The source of charged particles as claimed in claim 1, wherein said opposite walls are made of different materials, having different work functions, the wall at the higher temperature having the higher work function.

5. The source of charged particles as claimed in claim 1, wherein one of the opposite walls is provided with a lip, the arc discharge being thereby set up between thenedge of said lip and the other one of said opposite wa s.

6. The source of charged particles as claimed in claim 2, wherein said opposite walls are made of a common material.

7. The source of charged particles as claimed in claim 2, wherein said opposite walls are made of different materials, having different Work functions, the wall at the higher temperature having the higher work function.

8. The source of charged particles as claimed in claim 2, wherein one of the opposite walls is provided with a lip, the are discharge being thereby set up between the edge of said lip and the other one of said opposite walls.

9. A source of charged particles, comprising first means forming a cavity adapted to be heated including wall means, with electrons being adapted to be emitted from at least a portion of the internal surfaces of said wall means, second means for producing within said cavity a plasma of an ionizable substance by contact ionization, and third means for maintaining said cavity in nonisothermal conditions to produce an internal arc discharge in said cavity favoring volume ionization and thus reducing the number of neutral atoms, said third means being formed of materials having different work functions With the material having the higher work function being heated to a higher temperature.

10. A source of charged particles, comprising first means forming a cavity adapted to be heated including wall means, with electrons being adapted to be emitted from at least a portion of the internal surfaces of said Wall means, second means for producing within said cavity a plasma of an ionizable substance by contact ionization, and third means for maintaining said cavity in nonisothermal conditions to produce an internal arc discharge in said cavity favoring volume ionization and thus reducing the number of neutral atoms, said third means being effective to heat different parts of said wall means to different temperatures, the diiferent parts of the wall means being made of the same material.

References Cited UNITED STATES PATENTS JAMES W. LAWRENCE, Primary Examiner.

STANLEY D. SCHLOSSER, Examiner. 

1. A SOURCE OF CHARGED PARTICLES OF THE TYPE HAVING A HEATED CAVITY IN WHICH ELECTRONS ARE EMITTED FROM AT LEAST A PORTION OF THE INNER SURFACE OF THE CAVITY, WHILE A PLASMA OF AN IONIZABLE SUBSTANCE IS PRODUCED BY CONTACT IONIZATION WITHIN THE CAVITY, SAID CAVITY COMPRISING TWO MUTUALLY FACING, OPPOSITE WALLS, SAPCED APART BY AN INTERMEDIATE WALL AND HEATED TO TWO DIFFERENT TEMPERATURES, THE SPACING BETWEEN SAID OPPOSITE WALLS BEING SUFFICIENTLY SMALL TO ENABLE AN ARC DISCHARGE AT ELAST WITHIN A PORTION OF THE SPACE COMPRISED BETWEEN SAID OPPOSITE WALLS. 